hvmloader: Intel GPU passthrough, reverse OpRegion

hvmloader: Intel GPU passthrough, reverse OpRegion

The Intel GPU uses a two pages NVS region called OpRegion.
In order to get full support for the driver in the guest
we need to map this region.

This patch reserves 2 pages on the top of the memory in the
reserved area and mark this region as NVS in the e820. Then
we write the address to the config space (offset 0xfc) so the
device model can map the OpRegion at this address in the guest.

Signed-off-by: Jean Guyader <jean.guyader@eu.citrix.com>
Committed-by: Keir Fraser <keir@xen.org>